The present invention relates to an apparatus for clamping non-rotating work pieces which include an elongate portion or an elongate shank with polygonal, i.e., rectangular, cross sectional shape, preferably with regular polygonal cross sectional shape and particularly, but not exclusively, with substantially square cross sectional shape. Such work pieces may include electrode blanks which are mounted on an elongate shank, the shank thereof being clampingly received in the clamping apparatus, or electrode blanks which are of generally elongate shape to be directly received in the clamping apparatus, or finished electrodes having an elongate portion which is received in the clamping apparatus.
A clamping apparatus of the kind referred to is used to mount a tool for machining a work piece into a desired shape, e.g. an electrode for electro erosive machining of the work piece, or a work piece which has to be machined into an arbitrary shape and size, on a machining apparatus, e.g. a machine tool. It is understood that the expression "work piece" not only means a blank to be machined, but also a machining tool which has to be mounted on a machine tool with high positional accuracy. Obviously every tool once has been a blank piece which had to be machined into the desired shape. These reflections gain a pronounced importance in the field of electro erosive machining of metallic work pieces where the tools are electrodes. In this case, the "tool", i.e. the electrode, frequently is individually manufactured from a copper blank by turning and/or milling and/or grinding in order to be mounted on an electro erosive machining apparatus and to be used as the shape generating element.
Such electrode bodies, however, are tools which have to meet exceptionally high requirements as far as the dimensional accuracy is concerned, not only with reference to centricity, i.e. within the X- and Y-axes, but also with reference to its angular orientation around their central axis (Z-axis). Additionally, in many cases, a very high positional accuracy is required also along the Z-axis, i.e. the height position of the electrode with reference to the electro erosive machining apparatus.
To manufacture such an electrode body, a blank, e.g. a copper blank, is mounted in turn on a turning lathe and/or on a milling machine and/or on a grinding machine, as required, and thereby shaped into the desired final shape. Finally, the finished electrode is mounted on an electro erosive machining apparatus in order to machine a work piece with the extremely high precision common to the electro erosive machining process. In order to ensure the desired accuracy in machining, the shaping of the electrode body has to be performed within very narrow tolerance limits; this means that the mounting and clamping of the blank electrode body, of the partially machined electrode which has to be displaced from one machining station to the following one during the machining process, and of the finished electrode must be performed with very high precision, not only on the several machining stations, but also finally on the electro erosive machining apparatus itself.
Suitable coupling devices for mounting electrode bodies on electro erosive machining apparatuses are known in the art which ensure a highly precise centric mounting including a well defined angular orientation of the electrode. However, in many cases it is not possible to fix an electrode body or an electrode blank directly in such a coupling device, with the consequence that an auxiliary clamping apparatus is required. Besides the fact that such a clamping apparatus has to be very rigid and precisely manufactured, it is a primary requirement that it provides a repetitive positional accuracy which is as high as ever possible. This means with other words that a work piece, e.g. an electrode blank, during the manufacturing process or a finished electrode during subsequent erosion steps have to be removed from the clamping apparatus and re-inserted therein again many times, whereby the predetermined reference position in X-, Y- and, if required, also in Z-direction as well as with reference to the angular orientation always has to be taken up again with highest accuracy.